We find that inactivation of transglutaminase with neopentyl isocyanate generated from nitrosourea in tumor-bearing or pregnant mice results in the appearance of surface antigens which stimulated T-cells and these kill unmasked tumor cells and fetal cells. Our explanation is that inactive transglutaminase leaves the tumor's "altered self" intact' thus the "strangeness" of the tumor cell is free to stimulate the cellular immune system for the killing. Factor XIII (transglu-taminase)-deficient women not able to "mask" the fetus, abort it. With the addition of Factor XIII, the pregnancy is successful. This finding suggests that what we see with tumor-bearing syngenic mice might be applicble to autochtonous tumors as well.